Calculation of errors in three-winding voltage transformers

Voltage transformers having two secondary windings for connection to dual instrument burdens are discussed. After considering the vector diagrams under no-load and under load conditions, expressions are derived which enable the performance to be predetermined from the design data of the transformer. The influence of the burden on one of the secondary windings in modifying the errors at the terminals of the other secondary winding is investigated. From the analytical equations, further expressions are deduced to facilitate the estimation of the performance of a transformer when the results of standard tests are given. Finally, the calculated and measured errors of two transformers under various loading conditions are compared and are shown to agree.